Odd Mushroom

Effects

+1.7 tears.
+0.3 speed.
Makes Isaac's head smaller, which on its own is only a visual effect.
- If combined with other size downs, the Mushroom also affects his hitbox.
Reduces damage using the following formula: (Damage Ups from other items * 0.9) - 0.4.
(Added in Afterbirth) This item belongs to the Fun Guy set. Collecting three items from this set will transform Isaac into a walking mushroom.

There are two collectible items in the game named Odd Mushroom; you can tell them apart from the sprites.
This item affects damage differently than other items in the game. Instead of multiplying your damage stat, it directly alters the total added damage from other items. In practice, this means this item reduces your damage significantly if you have no items that grant extra damage (x0.72), but won't have as strong of an impact as you collect more 'damage up' items.

Odd Mushroom: Multiple copies of the item will grant a speed and fire rate boost with no additional decrease in damage.